The present invention relates to an electric circuit for a camera for controlling exposure and alarming vibrations of the camera.
Conventionally, an electric circuit for a camera for controlling exposure and alarming vibrations of the camera as shown in FIG. 1 is known. The electric circuit is set in its "vibration alarm mode" when a release button is depressed so lightly and that only contacts 1 and 2 of a switch SW1 are connected together and a power source E.sub.B is connected through these contacts to this electric circuit. In other worlds, with a trigger switch TRISW closed, the power supply voltage is divided by a series circuit comprising a light receiving element Rcds for exposure photometry and a resistor R1. That part of the voltage across the resistor R1 is applied to one input terminal of a comparator COMP to be compared with a reference voltage Vref, which is set by a variable resistor VR1 and is applied to the other input terminal of the comparator. The output of the comparator COMP is applied through the resistor R2 to a light emitting diode LED for warning the photographer when there is not enough light for taking a photograph. The output of the comparator COMP is low only when the voltage at the lower end of the light receiving element Rcds is lower than the reference voltage Vref at the arm of the variable resistor, or potentiometer, VR1. The electromagnet, or magnet, Mg and the light emitting diode LED are energized only when the output of the comparator COMP is low. Therefore, the light emitting diode should be energized only when there is not enough illumination on the scene to permit a photograph to be taken. The light emitting diode LED should be "off" when there is adequate scene illumination.
When the release button is depressed farther, all three of the contacts 1, 2, and 3 of the switch SW1 are connected together. This places the electric circuit in its "timer mode". Bringing the contacts 2 and 3 of the switch SW1 together short-circuits the resistor R1 so that an ordinary timer circuit is formed in which the resistive element is the light emitting element Rcds and the condenser is the condenser C1. The timing operation starts when the switch TRISW is opened, but as long as it is closed, and with the resistor R1 short-circuited, the voltage at the bottom of the light receiving element Rcds is substantially at the voltage of the negative terminal of the power supply E.sub.B. This is below the level of the reference voltage Vref, and so the output of the comparator COMP is low, which not only energizes the magnet Mg, but also causes the light emitting diode LED to be turned on, irrespective of the ambient light. As a result, as long as the circuit remains in this condition, the light emitting diode may well yield an incorrect indication of the available light on the scene which the photographer desires to photograph.
When the shutter is mechanically released, as by further actuation of the release button, the shutter blades open. Simultaneously, the trigger switch TRISW also opens. This causes the voltage oo begin to be integrated by the light receiving element Rcds and the condenser C1, which operates as an integrating condenser. When the integrated voltage at the common connection point between the condenser C1 and the light receiving element Rcds builds up sufficiently to be equal to the reference voltage Vref, the output of the comparator COMP changes from low to high, and current flow through the magnet Mg and through the light emitting diode LED is turned off and the shutter blades are closed, thus ending the exposure at a time determined by the timing circuit.
FIG. 2 shows a timing diagram of the above-mentioned operation. As can be seen from FIG. 2, when the construction of the release button that actuates the switch SW1 and the shutter is such that it can be stopped easily within the range in which the above-mentioned wrong display of the light emitting diode LED can be made, the light emitting diode LED may be lighted, irrespective of the surrounding lightness when the release button is stopped in the range. Therefore, it is preferable that the stroke of that release button in the range be as small as possible. However, when the release mechanism is of a sensor type, the whole stroke may be, for example, as small as 1.5 mm, although the normal stroke is in the range of 6 to 8 mm. In such case, it is difficult to adjust the above-mentioned stroke to be sufficiently small. Consequently, it is impossible to completely prevent the light emitting diode LED from being operated incorrectly.
In order to obviate such wrong display only, the following mechanism could be proposed, namely that the switch SW1 not be switched to the timer mode until after the mechanical release of the shutter. In this case, it is necessary that the magnet Mg be mechanically held before the trigger switch TRISW is opened, that is, before the integration is started. A period of about 10 msec will be necessary for the mechanical holding. Referring to FIG. 3, there is shown a time chart of this operation. As can be seen from FIG. 3, if there is sufficient light to prevent current from flowing through the magnet Mg and the light emitting diode LED during the "alarm mode" and if the above-mentioned period of time T1 in which the magnet Mg is mechanically held is less than 10 msec, there is an interval during which the magnet Mg is not being held, with the result that the magnet Mg works improperly. This could be prevented by providing a mechanical governor to be actuated until the trigger switch TRISW is turned on so that the period of time T1 is increased to more than 10 msec. However, the mechanism would become too complex.
Furthermore, some cameras are provided with synchronizing terminals to which or strobotron is connected. An X-contact is connected to the synchronizing terminals and, simultaneously with the mechanical release, the strobotron is lighted by closure of the X-contact. However, in such a system, the strobotron is lighted when the synchronizing terminal is connected to the strobotron even if there is enough light on the subject.
Furthermore, in an electro-shutter type camera having an X-contact, when part of the wiring of the X-contact is employed instead of the wiring of the electro-shutter, a power source line is employed. As shown in FIG. 4, an electric part, or circuit, P for the electro-shutter is connected to a battery E.sub.B, and a magnet Mg for the electro-shutter is connected between a power source positive line 4 and an output terminal of the electric part P. The X-contract 7 is connected between synchronizing terminals 5 and 6 by using part of the positive line 4. A strobotron circuit is shown in FIG. 5 in which the input side of a DC-DC converter 8 is connected to a battery 9, and a diode 10 and a main condenser 11 are connected in series between the output terminals of the DC-DC converter 8. A flash discharge tube 14 and the series circuit comprising a resistor 12 and a trigger condenser 13 are connected in parallel with a main condenser 11. A secondary circuit of a trigger transformer 15 is connected between one terminal of the flash discharge tube 14 and a trigger electrode. Furthermore, Terminals 16 and 17 are provided for connection to the synchronizing terminals 5 and 6, so that the primary circuit for the trigger transformer 15 can be connected in parallel with the trigger condenser 13 through the X-contact 7. Usually, the output of the battery 9 is converted to a high potential by the DC-DC converter 8 and charges the main condenser 11 through the diode 10 and, at the same time, charges the trigger condenser 13 through a resistor 12. When the terminals 16 and 17 are connected to the synchronizing terminals 5 and 6, the X-contact 7 is closed in synchronism with the shutter, whereby the trigger condenser 13 is discharged and the flash discharge tube 14 is triggered so as to be lighted by the discharging of the main condenser 11.
However, when the wiring of the X-contact is so arranged, normally the voltage across in the trigger condenser 13 is applied only to a primary side 15.sub.1 of the trigger transformer 15 when the X-contact 7 is closed as shown in FIG. 29. On the other hand, when the X-contact 7 is closed with part G disconnected, instead of being connected, as shown in FIG. 4, the voltage across the trigger condenser 13 is applied to the electric part P through the battery E.sub.B on the side of the camera as shown in FIG. 30. Furthermore, the inner resistance of the battery E.sub.B is so small that most of the voltage across the trigger condenser 13 is applied to the electric part P. Therefore, the electric part P is likely to be destroyed by application of a high voltage V1 which is beyond its breakdown voltage as shown by the solid line in FIG. 28. This is apt to occur particularly when it is necessary to separate the electric part P from the magnet Mg and the X-contact 7 and others. In other words, when the electric part P and the magnet Mg and the X-contact 7 are incorporated in one unit by connecting them by a connector, or when the magnet, the X-contact and the electric circuit are connected by contacts as in a mirror-tube-retractable type camera, the fact that the power source positive line section G is connected to the contacts means that improper contact is apt to occur.